Electronic devices such as computers, televisions, telephones and the like typically have several integrated circuits, each integrated circuit being formed on a semiconductor die and packaged in a protective package. The packages containing the integrated circuits are usually mounted on circuit boards with conductors printed or otherwise fabricated onto the circuit boards to interconnect the input and output terminals of the integrated circuit packages.
As the signaling rates between integrated circuits such as central processing units and other integrated circuits on the circuit board have increased to higher and higher frequencies, the frequency response of the circuit board interconnects may become uneven. As a consequence, high speed interconnects in lossy channels may suffer from unequal frequency-dependent loss, which can result in degraded signal integrity. For example, serial point-to-point interconnects such as those used in Peripheral Component Interconnect Express (PCIe) busses which typically utilize high-speed channels, may be susceptible to unequal frequency-dependent loss, particularly for the longer interconnects. The details of the Peripheral Component Interconnect Express standard are explained in greater detail in the Peripheral Component Interconnect Express Base 2.0 Specification, published Jan. 15, 2007.
One technique for reducing unequal frequency-dependent loss is to reduce the length of the interconnect between integrated circuit packages. However, due to system constraints, reduction of interconnect length may not be an available option. Repeater circuits may be used to reduce the length of an individual channel. However, repeater circuits may increase latency, circuit complexity and power consumption.
Another approach is to use a low loss material which can reduce unequal frequency-dependent loss. However, such low loss materials can significantly increase manufacturing costs.
Circuits known as “equalizers” may also be used to “equalize” the frequency dependent loss of a particular communication channel on a printed circuit board. One type of equalizer circuit is an active circuit which includes transistors which may be fabricated directly on the die of the integrated circuit itself. One such active equalizer circuit is known as a discrete-time filter and can be integrated in the TX (Transmitter) or RX (Receiver) portion of the integrated circuit. Some discrete-time filter active equalizer circuits have programmable equalization taps to provide the flexibility of handling a variety of loss profiles which may be encountered. However, active equalizer circuits often incur increased power consumption and latency.
In addition to active equalizer circuits, passive equalizer circuits using passive components such as inductors, capacitors and resistors are also known. Such equalizers, often referred to as continuous-time linear equalizers (CLEs), can equalize the frequency-dependent loss of an interconnect by suppressing low-frequency signals and passing high-frequency signals in a manner similar to that of a high-pass filter. The use of passive components can reduce power dissipation and latency due to the absence of active components. Passive equalizers may be fabricated on the die itself. Such on-die passive equalizers often can be tuned for particular applications through legging of resistors, capacitors or other components. It is also known to place passive equalizers on the printed circuit board using discrete components, that is, discrete resistors, capacitors and inductors.